Heat treatments of Inconel 718 nickel-based superalloy: A Review

Heat treatments are vital in obtaining suitable microstructural features leading to excellent high-temperature properties in Inconel 718 nickel-based superalloy. In this review article, the effects of various alloying elements of Inconel 718 like Fe, Cr, Al, Ti, Nb, Mo, Co, B, and C, and the relevant phases formed are discussed in context to the various heat treatments applied to the alloy. Heat treatments such as homogenization and age hardening applied to nickel-based superalloys in general and also specifically to Inconel 718 are reviewed in detail with respect to the processing routes employed, i.e., casting, wrought metallurgy, powder metallurgy and additive manufacturing. Some of the simulation techniques like the Scheil module in Thermo-Calc and DICTRA which are used for predicting the homogenization time and temperature are also discussed. The metallurgical aspects of nickel-based superalloys deal with their composition and property relationships, microstructure evolution, and various strengthening phases formed during heat treatments. The various precipitate phases of Inconel 718 i.e., gamma ', gamma '' and delta formed during the processing and heat treatments, and their contribution to the enhancement of high temperature and room temperature mechanical properties are also elucidated in detail. The effect of precipitate volume fraction, morphology, and size on the enhancement of high-temperature and room-temperature mechanical properties is reviewed in particular as per recently published reports in this field.